Well if it's a linear BEC you're wasting half that pack in heat for a start. - Linear BEC's are so 2007
You've also added another "point of failure" to your system and of the UBECs I've tested so far, not one has actually met the specifications published for it -- so that 3A UBEC will probably shut-down if you get a stalled servo or even a couple of stiff linkages plus some aerodynamic loads. Flight Pack + UBEC = 2 points of failure. Flight Pack + Life Pack = 2 Points of failure
If you over-discharge the lipo it's toast (A123s will simply take a fresh charge with no worries). - Lipos have a higher energy density than an A123 (in can construction) so for the same size/weight you can fit more Lipo MAh, reducing the chance of over discharging anyway.
If you dork it hard enough, the battery may catch fire (A123s don't do that). If you dork your plane hard enough to set a lipo on fire, the plane is likely so Bleeped that the best idea for it is a fire anyway.
A lipo has about 200-300 charge/discharge cycles in it -- an A123 is good for five to eight times as many. A modern low C rated lipo running at naff all current draw as a Rx pack, properly stored and charged should last 500+ cycles easy
There are lots of reasons why A123s are a far better option than a lipo/regulator setup. There are some difference between the two, but LOTS might be an exageration

If it was possible to plug a little doofa into your slimers gas tank and run your rx directly off that, im sure you would!

I like those little 3A turnigy jobbies, used them in heaps of stuff and never had a problem, but I have never tested them
Be interested in seeing a test on those RCMR, and the CC10A SBEC too (similar size)
BTW Highest servo current draw is usually seen when travel is reversed, ie full one way then bang full the other way. Stall current is usually around half that.